This invention relates to a rear projection color TV using plural cathode ray tubes placed horizontally, and is directed particularly to a high performance rear projection screen with wide audience coverage and free of color-shifts or color shading.
FIG. 1 shows a fundamental configuration of optical system used in a conventional rear projection color TV. In FIG. 1, reference numeral 1 designates a red CRT the image on which is projected by the projection lens 3 onto the screen 5. The screen 5 is composed of the Fresnel lens 6 and the lenticulars 7. The Fresnel lens 6 serves to microscopically transform the input diverging light from the projection lens into the output parallel light approximately normal to the screen. The lenticulars 7 serve to microscopically diffuse the light so as to form the image on the screen. The half-gain-angle of the diffuser determines the audience coverage.
FIG. 2 shows the desirable audience coverage. In ordinary use, .+-.8.degree. for vertical direction and .+-.45.degree. degree for horizontal direction are the desirable amount.
FIG. 3 shows the same relationship in a graphical representation, where curve 9 represents the horizontal directivity and curve 10 represents the vertical directivity.
In FIG. 1, a converging angle .epsilon. is about 6.degree.-8.degree. in the ordinary case. This angle .epsilon. is liable to cause color shifts in a reproduced picture; that is, when view from the direction A, the red color tends to be stronger than the blue color; when viewed from the direction B, the blue color tends to be stronger than the red color. This color shift or color-shading greatly deteriorates the picture quality.
Few approaches have been successful in the past in conquering this color shift.
There exists one idea which is disclosed in U.S. Pat. No. 4,054,907, where a construction shown in FIG. 4 is used for lenticulars 7 in FIG. 1. This idea assumes and is suitable for the vertically-in-line configuration of 3 tubes and 3 projection lenses although the horizontally-in-line configuration has been the more popular case in actual cases in these years and although no actual approach has been successful in conquering color shifts in the horizontal-in-line case because of the wide audience coverage required therein. In FIG. 4, numeral 11 designates a vertically diffusing cylindrical lenticular lens of which curvature radius r1 is 1 mm and the period P1 is 0.8 mm, which produces a vertical audience coverage of .+-.12.degree.. Numeral 12 designates a color-shift-compensation lenticular lens of which the curvature radius and period is the same as each respective lenticular lens 11. Numeral 13 designates a horizontally-diffusing cylindrical lenticular lens of which radius curvature r3 is 0.4 mm and the period P3 is 0.6 mm, which produces a horizontal audience coverage of approximately .+-.27.degree.. Numeral 14 designates a block stripe which reduces the reflectivity of the exit surface to the ambient illumination and which enhances the picture contrast.
FIG. 5 shows the ray-trace and principle of the lenticular lenses 11 and 12. In FIG. 5, a solid line designates a blue beam and a dotted line designates a red beam. As can be seen from FIG. 5, the beam landing on the center position of the input lenticular surface is transformed into the parallel output beam normal to the screen surface. Although the principle explained above pursuant to U.S. Pat. No. 4,054,907 holds very well with the specific small audience coverage and with the specific vertically-in-line configuration of 3-tubes mentioned above, it has its own limitation in the extent of the application suitable for actual cases.
In regard of practical cases, it has three disadvantages as follows.
(1) Vertically-in-line configuration of 3 tubes is difficult to build into a compact cabinet suitable for home-use because the piled height of the 3 tubes and 3 projection lenses tends to be too big as shown in FIG. 6 compared with the conventional horizontally-in-line configuration.
(2) In case of a horizontally-in-line configuration of 3 tubes, a diffusing angle of .+-.12.degree. as in the case of FIG. 5 is not sufficient. More than .+-.35.degree. is required for horizontal audience coverage. FIG. 7 shows what happens when the principle above is applied to the wider audience coverage. To attain horizontal audience coverage of .+-.40.degree., the period to radius ratio is required to be increased up to 1.8. In FIG. 7, reference numeral 15 designates the directivity for blue color and numeral 16 the directivity for red color. The ratio of directivity of blue to red is approximately 1.6 when observed from the side, which results in an unbalanced percentage of as much as 60%, which is worse than the desirable limit of 30%. The reason why this big unbalance is caused is attributed to the unbalance between the upper half and lower half of the input luminous flux: ##EQU1## in FIG. 7.
(3) Horizontal audience coverage is as narrow as .+-.27.degree. as shown in FIG. 8 in case of the cylindrical lens configulation described above. The reason is that this narrow pass angle is inherent to the cylindrical lens configuration of lenticular 13 in FIG. 4. FIG. 8 shows the theoretical horizontal directivity calculated by the inventor pursuant to the method (equation 2) disclosed later in explanation of the invention, and it holds very well with empirical results so far obtained.
No modification in radius nor in period within the category a single cylindrical lens can achieve broader audience coverage than that shown in FIG. 8.